Explain N-poles and E-poles?
This ,and more, is on the Topology Guides site:


  E-poles are the most common pole type and consist of five edges intersecting at a single vertex. E-poles are most notorious for appearing when extruding faces on a mesh and for forming unwanted flat “corners” within the topology when edge loops meet or turn.  E-poles are also what form concave corners in hard surface models.


and


  N-poles are vertices that consist of three intersecting edges. This type of pole is far less common, but often appearing around poles or inset parts of a mesh. In organic modeling, this pole is often known as the “nose” pole, since N-poles are often necessary for modeling the base of the nose. N-poles are also what form the corner of a cube or most other convex corners.